1.表信息(category)
第一条sql语句:
SELECT cname AS NAME FROM category;
第二条sql语句
SELECT (SELECT COUNT(cid) FROM category) AS NAME;
第三条sql语句
SELECT (SELECT cname FROM category) AS NAME;
这里有点不懂:
1.第一条sql和第三条sql为什么第一条可以查询多条记录,第三条不行?
2.为什么第三条sql不能查询多条记录?
3.第二和第三条说明select (子查询) as 别名;这样的sql语句时,sql子查询就只能有一条记录,为什么是这样的?
4.第三条sql的意思不是给子查询中的cname字段取别名吗?如果不是,那是什么意思?
问题解决:
不能给多行取别名,第一条sql语句是给一列取别名,第二条是给一行取别名,第三条是给多行取别名所以失败了!
疑问来源:
177. Nth Highest Salary
Write a SQL query to get the nth highest salary from the Employee table.
+----+--------+ | Id | Salary | +----+--------+ | 1 | 100 | | 2 | 200 | | 3 | 300 | +----+--------+
For example, given the above Employee table, the nth highest salary where n = 2 is 200. If there is no nth highest salary, then the query should return null.
+------------------------+ | getNthHighestSalary(2) | +------------------------+ | 200 | +------------------------+
BEGIN
DECLARE M INT;
SET M=N-1;
RETURN (
# Write your MySQL query statement below.
SELECT
IFNULL(
(SELECT DISTINCT Salary
FROM Employee
ORDER BY Salary DESC
LIMIT 1 OFFSET M),
NULL) AS SecondHighestSalary
);
END